Vesta is the brightest asteroid visible from Earth. Its maximum distance from the Sun is slightly farther than the minimum distance of Ceres from the Sun,[22] though its orbit lies entirely within the Cererian orbit.[23]

NASA's Dawn spacecraft entered orbit around Vesta on 16 July 2011 for a planned one-year exploration, and what is known about Vesta will be refined and extended as data from Dawn is received, analyzed and published.

Discovery

Heinrich Olbers discovered Pallas in 1802, the year after the discovery of Ceres. He proposed that the two objects were the remnants of a destroyed planet. He sent a letter with his proposal to the English astronomer William Herschel, suggesting that a search near the locations where the orbits of Ceres and Pallas intersected might reveal more fragments. These orbital intersections were located in the constellations of Cetus and Virgo.[24] Olbers commenced his search in 1802, and on 29 March 1807 he discovered Vesta in the constellation Virgo — a coincidence, as Ceres, Pallas, and Vesta are not fragments of a larger body. As the asteroid Juno had been discovered in 1804, this made Vesta the fourth object to be identified in the region that is now known as the asteroid belt. The discovery was announced in a letter addressed to German astronomer Johann H. Schröter dated 31 March.[25] Because Olbers already had credit for discovering a planet, Pallas (at the time, the asteroids were considered to be planets), he gave the honor of naming his new discovery to German mathematician Carl Friedrich Gauss, whose orbital calculations had enabled astronomers to confirm the existence of Ceres, the first asteroid, and who had computed the orbit of the new planet in the remarkably short time of 10 hours.[26][27] Gauss decided on the Roman virgin goddess of home and hearth, Vesta.[28]

Nomenclature

Vesta was the fourth asteroid to be discovered, hence the number 4 in its formal designation. The name Vesta, or national variants thereof, is in international use with two exceptions, Greece and China. In Greek the name adopted was the Hellenic equivalent of Vesta, Hestia (4 Εστία); in English, that name is used for 46 Hestia (Greeks use the name "Hestia" for both, with the asteroid numbers used for disambiguation). In Chinese, Vesta is called the 'hearth-god(dess) star', 灶神星 zàoshénxīng, in contrast to the goddess Vesta, who goes by her Latin name.[29]

When Olbers discovered Vesta, Ceres, Pallas, and Juno were classified as planets and each had its own planetary symbol. Vesta was likewise classified as a planet, and along with its name, Gauss designed an appropriate planetary symbol, ⚶, the altar of the Vesta with its sacred fire.[30][31] In Gauss's conception this was drawn ; in its modern form it is .[note 1]

After the discovery of Vesta, no further objects were discovered for 38 years, and the Solar System was thought to have eleven planets.[34] However, in 1845 new asteroids started being discovered at a rapid pace, and by 1851 there were fifteen, each with its own symbol, in addition to the seven major planets. It soon became clear that it would be impractical to continue inventing new planetary symbols indefinitely, and some of the existing ones proved difficult to draw quickly. That year the problem was addressed by Benjamin Apthorp Gould, who suggested numbering asteroids in their order of discovery, and placing this number in a disk (circle) as the generic symbol of an asteroid. Thus the fourth asteroid, Vesta, acquired the generic symbol ④. This was soon coupled with the name into an official number–name designation, ④ Vesta, as the number of minor planets increased. By ca 1858, the circle had been simplified to parentheses, (4) and (4) Vesta, which was easier to typeset. Other punctuation such as 4) Vesta and 4, Vesta was also used, but had more or less completely died out by 1949.[35] Today either (4) Vesta or more commonly 4 Vesta is used.

Early measurements

Photometric observations of the asteroid Vesta were made at the Harvard College Observatory in 1880–1882 and at the Observatoire de Toulouse in 1909. These and other observations allowed the rotation rate of the asteroid to be determined by the 1950s. However, the early estimates of the rotation rate came into question because the light curve included variations in both shape and albedo.[36]

Early estimates of the diameter of Vesta ranged from 383 (in 1825) to 444 km. E.C. Pickering produced an estimated diameter of 513 ± 17 km in 1879, which is close to the modern value for the mean diameter, but the subsequent estimates ranged from a low of 390 km up to a high of 602 km during the next century. The measured estimates were based on photometry. In 1989, speckle interferometry was used to measure a dimension that varied between 498 and 548 km during the rotational period.[37] In 1991, an occultation of the star SAO 93228 by Vesta was observed from multiple locations in the eastern United States and Canada. Based on observations from 14 different sites, the best fit to the data is an elliptical profile with dimensions of about 550 km × 462 km.[38]

Vesta became the first asteroid to have its mass determined. Every 18 years, the asteroid 197 Arete approaches within 0.04 AU of Vesta. In 1966, based upon observations of Vesta's gravitational perturbations of Arete, Hans G. Hertz estimated the mass of Vesta as (1.20 ± 0.08) × 10−10solar masses.[39] More refined estimates followed, and in 2001 the perturbations of 17 Thetis were used to estimate the mass of Vesta as (1.31 ± 0.02) × 10−10 solar masses.[40]

Physical characteristics

The IAU 2006 draft proposal on the definition of a planet listed Vesta as a candidate.[41] Vesta is shown fourth from the left along the bottom row.

Vesta is the second-most-massive body in the asteroid belt,[42] though only 28% as massive as Ceres.[17] It orbits in the inner asteroid belt interior to the Kirkwood gap at 2.50 AU. It has a differentiated interior,[18] and is similar to 2 Pallas in volume (to within uncertainty) but about 25% more massive.[42]

Temperatures on the surface have been estimated to lie between about −20 °C with the Sun overhead, dropping to about −190 °C at the winter pole. Typical daytime and nighttime temperatures are −60 °C and −130 °C, respectively. This estimate is for 6 May 1996, very close to perihelion, while details vary somewhat with the seasons.[7]

The eastern and western hemispheres show markedly different terrains. From preliminary spectral analyses of the Hubble Space Telescope images,[46] the eastern hemisphere appears to be some kind of high-albedo, heavily cratered "highland" terrain with aged regolith, and craters probing into deeper plutonic layers of the crust. On the other hand, large regions of the western hemisphere are taken up by dark geologic units thought to be surface basalts, perhaps analogous to the lunar maria.[46]

Rheasilvia crater

The most prominent of these surface features is an enormous crater 505 kilometres (314 mi) in diameter centered near the south pole.[43][47] The Dawn science team has named it Rheasilvia, after the mother of Romulus and Remus and a mythical vestal virgin.[48] Its width is 90% the diameter of Vesta. The floor of this crater is about 13 kilometres (8.1 mi) below, and its rim rises 4–12 km above the surrounding terrain, with total surface relief of about 25 km. A central peak rises 23 km above the lowest measured part of the crater floor and the highest measured part of the crater rim is 31 km above the crater floor low point. It is estimated that the impact responsible excavated about 1% of the volume of Vesta, and it is likely that the Vesta family and V-type asteroids are the products of this collision. If this is the case, then the fact that 10-km fragments have survived bombardment until the present indicates that the crater is at most only about 1 billion years old.[46] It would also be the original site of origin of the HED meteorites. In fact, all the known V-type asteroids taken together account for only about 6% of the ejected volume, with the rest presumably either in small fragments, ejected by approaching the 3:1 Kirkwood gap, or perturbed away by the Yarkovsky effect or radiation pressure. Spectroscopic analyses of the Hubble images have shown that this crater has penetrated deep through several distinct layers of the crust, and possibly into the mantle, as indicated by spectral signatures of olivine.[43]

At the center of Rheasylvia is a large central massif, that is 20 to 25 kilometres (12–16 mi) high and 180 kilometres (110 mi) wide.[49] The crater overlies an older one, Veneneia, that at 395 kilometres (245 mi) across is almost as large.[47]

Other craters

Several large craters about 150 kilometres (93 mi) wide and 7 kilometres (4.3 mi) deep are also present. A dark albedo feature about 200 kilometres (120 mi) across has been named Olbers in honour of Vesta's discoverer, but it does not appear in elevation maps as a fresh crater would. Its nature is presently unknown; it may be an old basaltic surface.[50] It serves as a reference point with the 0° longitudeprime meridian defined to pass through its center.

"Snowman craters"

The "snowman craters" is an informal name given to a group of three adjacent craters in Vesta's northern hemisphere. Their official names from largest to smallest (west to east) are Marcia, Calpurnia and Minucia.

Equatorial grooves

The southern equatorial region of Vesta is characterized by a series of concentric grooves, such as Divalia Fossa. These are thought to be compression fractures from the impact that created Rheasilvia crater. If they are continuous features, they would be one of the longest chasms in the Solar System, nearly as long as Ithaca Chasma on Tethys.

Vesta is the only known intact asteroid that has been resurfaced in this manner. Because of this, some scientists refer to Vesta as a protoplanet, rather than an asteroid.[57] However, the presence of iron meteorites and achondritic meteorite classes without identified parent bodies indicates that there once were other differentiated planetesimals with igneous histories, which have since been shattered by impacts.

On the basis of the sizes of V-type asteroids (thought to be pieces of Vesta's crust ejected during large impacts), and the depth of the south polar crater (see below), the crust is thought to be roughly 10 kilometres (6 mi) thick.[59]

Because a number of meteorites are believed to be Vestian fragments, Vesta is currently one of only six identified Solar System bodies for which we have physical samples, the others being Mars, the Moon, comet Wild 2, 25143 Itokawa, and Earth itself.[21]

Exploration

In 1981, a proposal for an asteroid mission was submitted to the European Space Agency (ESA). Named the Asteroidal Gravity Optical and Radar Analysis (AGORA), this spacecraft was to launch some time in 1990–1994 and perform two flybys of large asteroids. The preferred target for this mission was Vesta. AGORA would reach the asteroid belt either by a gravitational slingshot trajectory past Mars or by means of a small ion engine. However, the proposal was refused by the ESA. A joint NASA–ESA asteroid mission was then drawn up for a Multiple Asteroid Orbiter with Solar Electric Propulsion (MAOSEP), with one of the mission profiles including an orbit of Vesta. NASA indicated they were not interested in an asteroid mission. Instead, the ESA set up a technological study of a spacecraft with an ion drive. Other missions to the asteroid belt were proposed in the 1980s by France, Germany, Italy and the United States, but none were approved.[60] Exploration of Vesta by fly-by and impacting penetrator was the second main target of the first plan of the multiaimed Soviet Vesta mission, developed in cooperation with European countries for realisation in 1991–1994 but canceled due to the Soviet Union disbanding.

In the early 1990s, NASA initiated the Discovery Program, which was intended to be a series of low-cost scientific missions. In 1996, the program's study team recommended as a high priority a mission to explore the asteroid belt using a spacecraft with an ion engine. Funding for this program remained problematic for several years, but by 2004 the Dawn vehicle had passed its critical design review.[61]

It launched on 27 September 2007, as the first space mission to Vesta. On 3 May 2011, Dawn acquired its first targeting image 1.2 million kilometers from Vesta.[62] On 16 July 2011, NASA confirmed that it received telemetry from Dawn indicating that the spacecraft successfully entered Vesta's orbit.[63] It is scheduled to orbit the asteroid for one year, until July 2012.[64] Dawn's arrival coincides with late summer in the southern hemisphere of Vesta, with the large crater at Vesta's south pole (Rheasilvia) in sunlight. Because a season on Vesta lasts eleven months, the northern hemisphere, including anticipated compression fractures opposite the crater, will become visible to Dawn's cameras before it leaves orbit.[65]

NASA/DRL released imagery and summary information from a high-altitude orbit, including a two-minute video, in September 2011. Much more detailed imagery was planned to be obtained, from a lower orbit, beginning in October 2011.[66]

Scientists will use Dawn to calculate Vesta's precise mass based on gravitational interactions. This will allow scientists to refine the mass estimates of the asteroids that are in turn perturbed by Vesta.[61]

Observations from Earth orbit

Albedo and spectral maps of 4 Vesta, as determined from Hubble Space Telescope images from November 1994

Elevation map of 4 Vesta, as determined from Hubble Space Telescope images of May 1996

Less favorable oppositions during late autumn 2008 in the Northern Hemisphere still had Vesta at a magnitude of from +6.5 to +7.3.[69] Even when in conjunction with the Sun, Vesta will have a magnitude around +8.5; thus from a pollution-free sky it can be observed with binoculars even at elongations much smaller than near opposition.[69]

2010–2011

In 2010, Vesta reached opposition in the constellation of Leo on the night of 17–18 February, when it was about magnitude 6.1,[70] a brightness that makes it visible in binocular range but not for the naked eye. Under perfect dark sky conditions where all light pollution is absent it might be visible to an experienced observer without the use of a telescope or binoculars. Vesta came to opposition again on 5 August 2011, in the constellation of Capricornus at about magnitude 5.6.[70][71]

^On 10 February 2009, during Ceres perihelion, Ceres was closer to the Sun than Vesta, since Vesta has an aphelion distance greater than Ceres' perihelion distance. (2009-02-10: Vesta 2.56AU; Ceres 2.54AU)